Aqueous solutions of folic acid or salts thereof have only a limited stability in the presence of oxygen. In the presence of buffered solutions, the folic acid molecule is cleaved within a more or less short period of time into biologically inactive fragments, too (cf. B. Koft, G. Sevag in J. Am. Chem. Soc. 71, 3245 (1949)). For initiation of this decomposition of the folic acid a high humidity of the air is already sufficient (cf. F. Y. Triptet, U. W. Kesselring in Pharm. Acta Helv. 50 (10), 318-322 (1975)).
From British Pat. No. 725,683 it is known that aqueous solutions of alkali metal salts of folic acid may be stabilized by the addition of alkali metal salts of ethylenediaminetetraacetic acid. However, in practice it has turned out that the addition of the alkali metal salts of ethylenediaminetetraacetic acid has only a low stabilizing effect against oxygen contained in the air. Further, the use of ethylenediaminetetraacetic acid is physiologically problematic, when the vitamine preparation is to be used for the feeding of animals or for related application fields.
In cases where aqueous folic acid solutions have to be stored in the presence of air for extended periods of time, e.g. for several weeks, as is the case, for example, when they are used in biological sewage treatment plants, a significant decrease in bioactivity of the aqueous folic acid solutions had to be put up with, when using previous methods of stabilization.
It is the problem underlying the present invention to show a way to impart storage stability to aqueous solutions of folic acid and/or of salts thereof in the presence of oxygen, i.e. to impart a longer lasting stability to them.